In general, a liquefied gas tanker ship, e.g. for conveying petroleum or natural gas, comprises tanks for storing the liquefied gas at atmospheric pressure and at a temperature of about −160° C. Although the tanks containing the cargo are insulated, a fraction of the cargo evaporates on a continuous basis, typically of the order of 0.1% to 0.3% per day, because of the heat that penetrates through the insulation.
While the tanker is under way, the liquefied gas vapor is advantageously used as fuel for propelling the ship. When the liquefied gas vapor is not used for propulsion purposes, or when there is an excess of such vapor, regulations require the excess gas to be eliminated by being burnt, or else by being reliquefied, since any dumping of the liquefied gas vapor directly into the atmosphere is prohibited.
Installing a reliquefier on board the ship is generally very complex and expensive, so the solution that is implemented in most circumstances has therefore been to burn off the excess gas vapor. It is therefore necessary to have on board the ship means for incinerating the gas in complete safety, in particular without allowing a bare flame to appear, while also exhausting combustion gas at a temperature of less than about 450° C., as permitted by the regulations in force.
To satisfy these constraints, liquefied gas tanker ships have until now been fitted with a steam turbine propulsion system in which the liquefied gas vapor is burnt in the boiler of the propulsion system. The steam produced by the boiler is directed either directly to the turbine for propelling the ship or else to a steam/water condenser, assuming that the amount of steam exceeds the energy requirements of the ship. Under such circumstances, the boiler serves simultaneously as a steam generator for the propulsion system and as an incinerator of excess liquefied gas vapor when the ship's energy requirements are small.
Unfortunately, that type of steam turbine propulsion for liquefied gas tanker ships nevertheless suffers from major drawbacks, and in particular:                low efficiency, in particular efficiency that is lower than gas diesel propulsion systems, gas turbine systems, or even slow or heavy diesel fuel systems;        large size, diminishing the volume available for the cargo, for given volume of ship's hull; and        unusual propulsion technology, which can lead to difficulties of maintenance and of crew training.        
That type of propulsion is thus being replaced at present by diesel engine propulsion systems burning gas, by gas turbines, or by diesel engines operating on heavy fuel. Unfortunately, those propulsion systems cannot be used for incinerating excess liquefied gas vapor. It is therefore necessary to associate them with a specific device for incinerating such gas.
Even with slow diesel engines that do not use natural gas vapor as fuel and that are generally coupled to a reliquefier, one or more incinerators are still required by classification companies in order to perform two functions: the first function relates to eliminating the nitrogen-rich portion of the natural gas vapor that it is not economically viable to reliquefy; and the second function relates to eliminating all of the vapor when the reliquefier(s) is/are out of order.
Thus, even for ships that do not use steam propulsion that can burn the vapor escaping from the tanks, classification companies still require an additional device for incinerating the vapor.
FIG. 3 is a highly diagrammatic view of a prior art on-board device 101 for incinerating gas or vapor.
The device 101 comprises a combustion chamber 103 and a chimney 111. The combustion chamber 103 comprises a heater body 105 having one or more burners 147 placed in the enclosure of the combustion chamber 103 which is generally of dimensions larger than those of the chimney 111. Thus, the combustion chamber 103 is connected to the chimney 111 by a connection piece 116 via a flexible coupling 108 for compensating the effects of expansion.
In order to bring the temperature of the gas 113 at the outlet from the chimney 111 to an acceptable temperature, the combustion chamber 103 is fed with excess air so that the hot gas from the flames 131 of the burners 147 is mixed with cool air. This cool combustion-and-dilution air is forced into the combustion chamber 103 by fans 107a, 107b driven by motors 113a, 113b. 
In order to force mixing between the hot gas and the cool air, turbulators 135 are optionally placed in the combustion chamber 103 or in the chimney 111. These turbulators 135 need to be made of refractory materials, e.g. refractory steels or bricks that are expensive to purchase and to maintain.
The use of ambient air for simultaneously ensuring combustion and dilution can sometimes lead to considering separating these two functions by means of two series of fans. A first series of fans 107a and 107b is dedicated mainly to supplying combustion air, while a second series of fans 108 driven by motors 114 is dedicated to supplying dilution air. The injection point of the cool air delivered by these fans 108 is generally located in the high portion of the combustion chamber 103 thus making it possible, amongst other things, to reduce head losses.
The burners 147 are ignited by pilot flames 132 fed by a separate circuit for gas or fuel oil. This leads to extra expense, both at purchase and during maintenance, and the use of an additional fuel can lead to a fire hazard. The pilot flames 132 are themselves ignited by electrical spark plugs 171.
A diaphragm 151 is optionally placed level with the burners 147 to optimize the distribution of air around them and to generate turbulence for “catching” the flames 131.
In order to avoid, amongst other things, any risk of the crew suffering burns, the combustion chamber 103 and the chimney 111 are lined with thermal insulation 104 either on the inside or on the outside.
For safety reasons, the line 157 feeding gas to the burners 147 is fitted with two cut-off valves 161 and 163 which can be caused to close in the event of flames 131 not being detected at the burners 147. In addition, in order to satisfy safety essentials, a third valve 173 is placed to direct any gas that is trapped between these two valves 161 and 163 to the vent.
The flow rate of the gas sent to the incinerator 101 for being treated therein is usually controlled by a regulator valve 159.
In order to handle transients in the gas line 157, e.g. when changing the operating speed of the engines or of a reliquefier, a buffer tank 181 can optionally be placed upstream from the valves 159, 161, and 163. The buffer tank 181 serves to damp pressure variations in the gas line 157, making it possible, for example, to launch the sequence of igniting the incinerator 101 before being able to open the valves 159, 161, and 163 in order to burn off the excess gas in the gas line 157.
The buffer tank 181 operates between a minimum pressure and a maximum pressure for the line feeding the engines or the reliquefier, and that constitutes a relatively small range of pressures, of the order of a few hundreds of kilopascals (kPa). The buffer tank 181 must therefore have very considerable volume, typically several tens of cubic meters (m3), thus presenting a factor of cost and of bulk.
In addition to burning off the natural gas vapor coming from the tanks of a ship that is not consumed by the propulsion system, or that is not reliquefied by the reliquefier, the incinerators on board methane tankers are also used during maintenance operations for eliminating mixtures of natural gas and inert gas.
When maintenance operations are necessary inside the tanks, the natural gas they contain must be replaced initially by an inert gas and then by air.
After emptying out the last of the liquefied natural gas cargo, the tanks still full of natural gas vapor are initially heated progressively by causing a portion of the gas to circulate in a closed circuit through heat exchangers. In order to maintain a constant pressure in the tanks during this heating operation, a fraction of the vapor is burnt off in the ship's propulsion system or by the incinerator 101.
Once the temperature in the tanks is close to ambient temperature, a mixture of nitrogen and carbon dioxide gas supplied by the inert gas generator of the ship is injected into the tanks to expel the natural gas vapor. The mixture of natural gas vapor and of inert gas is evacuated to the incinerator 101 in order to be burnt therein. Since the methane content of this mixture can be low, particularly towards the end of the operation, the auxiliary support flames (pilot flames 132), generally burning a different fuel such as fuel oil, are used to ensure that the mixture burns, thus increasing the risk of fire.
Furthermore, patent DE10211645 describes a gas incinerator installed on a ship and having two combustion chambers and a chimney. The combustion chambers are fed with combustion air via radial fans or blowers and with dilution air via radial fans. The connection between the combustion chambers and the chimney is located at the outlet from said combustion chambers and is therefore at the same temperature as the hot gas evacuated by the chimney, thereby running the risk, in the event of rupture, of hot gas leaking into the premises where the incinerator is located.
Furthermore, in addition to the risk of hot gas leaking, the incinerator devices of the prior art present several other drawbacks.
Such devices are bulky and present high levels of head loss, requiring fans and motors of significant power.